Nonideal optical response of liquid crystal variable retarders and its impact on their performance as polarization modulators

Liquid crystal variable retarders (LCVRs) will be used for the first time in a space instrument, the Solar Orbiter mission of the European Space Agency, as polarization states analyzers (PSAs). These devices will determine the Stokes parameters of the light coming from the Sun by temporal polarization modulation, using the so-called modulation matrix O. This is a matrix constituted by the first rows of properly selected PSA Mueller matrices. Calibrating a space instrument, in particular, finding O, is a critical point because in a spacecraft there is no possibility of physical access. Due to the huge difficulty in calibrating the complete instruments in all possible scenarios, a more complete calibration of the individual components has been done in ground in order to make extrapolations to obtain O in-flight. Nevertheless, apart from the individual calibrations, the experimental errors and nonideal effects that inhibit the system to reach the designed and theoretical values must be known. In this work, description and study of these effects have been done, focusing on the nonideal effects of the LCVRs and the azimuthal misalignments between the optical components of the PSA during the mechanical assembly. The Mueller matrix of a representative LCVR has been measured and mathematically decomposed by logarithm decomposition, looking for values of circular birefringence and fast axis angle variations as a function of voltage. These effects, in the absence of other nonidealities, affect the polarimetric performance, reducing the polarimetric efficiencies in some cases until 11%. Nevertheless, in this case, they are negligible if compared to the other nonideality studied, which are the azimuthal misalignments between the PSA optical components. The study presented in this work is key to extrapolate the PSA O matrix if the expected instrumental set-point temperatures are not reached in flight and can be used for the design and implementation of other polarimetric instruments

Retrieving physical information of depolarizing systems

Light interaction with material systems may introduce depolarization to the incident light. This phenomenon comes from multiple scattering processes that take place inside the media and strongly depends on the particle characteristics. In the case of botany, plant leaves can be understood as depolarizing systems. A non-contact method to analyze these samples consist of illuminating them with well-known polarized light and study the scattered light to retrieve the physical characteristics of the sample. This physical study can be done by measuring the Mueller matrix of samples, in which the physical information of samples is encoded in their 16 elements and further mathematical treatment is required to extract the information. In the case of scattering systems, the depolarization content carries very valuable information but it is usually not inspected in the botanic field. A way to study depolarized content is by determining the so-called depolarization index P¿, which gives an overall measure of the degree of depolarization of a system but it does not measure possible anisotropic dependence of the depolarization. For instance, a depolarizer equally depolarizing any fully polarized input polarization or a depolarizer that depolarizes them in a strongly heterogenous way, may lead to the same P¿ value. In contrast, the Indices of Polarimetric Purity (IPP) are a group of metrics that further synthesize the depolarizing content, taking into account the anisotropic depolarization. In this work, we describe the main physical characteristics of samples achieved by using these IPP through plant samples. Moreover, we show how IPP highlights some structures hidden in regular intensity measurements, highlighting the potential of these metrics for botanical applications

Experimental validation of Mueller matrix differential decomposition

International audienceMueller matrix differential decomposition is a novel method for retrieving the polarimetric properties of general depolarizing anisotropic media [N. Ortega-Quijano and J. L. Arce-Diego, Opt. Lett. 36, 1942 (2011), R. Ossikovski, Opt. Lett. 36, 2330 (2011)]. The method has been verified for Mueller matrices available in the literature. We experimentally validate the decomposition for five different experimental setups with different commutation properties and controlled optical parameters, comparing the differential decomposition with the forward and reverse polar decompositions. The results enable to verify the method and to highlight its advantages for certain experimental applications of high interest